Controllable function potentiometer



J. Vcx. HEsELwooD 2,913,694

coNTRoLLABLE FUNCTION POTENTIOMETER 5 Sheets-Sheet l QWHQISQWQ T? s ff o :v

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Nov. 17, 1959l med Jan. 2o, 195e NOV- 17, 1959 J. c. HEsELwooD CONTROLLABLE FUNCTION POTENTIOMETER 5 Sheds-Sheet 2 Filed Jan. 20, 1956 HIIIHIIIIIIHI lx D 0 Nov. 17, 1959 J. c. HEsELwooD CONTROLLABLE FUNCTION POTENTIOMETER 5 Sheets-Sheet 3 Filed Jan. 20, 1956 NOV. 17, 1959 J, C, HESELWOQD 2,913,694

CONTROLLABLEFUNCTION POTENTIOMETER Filed Jan. 20. 1956 5 Sheets-Sheet. 4

Nov. 17, 1959 J. c'. HEsELwooD coNTRoLLABLE FUNCTION POTENTIOMETER 5 Sheets-Sheet 5 Filed Jan. 20, 1956 United States Patent() CONTROLLABLE FUNCTION POTENTIOlWETER lJames C. Heselwood, Havertown, Pa., assignor to Bethlehem Steel Company, Bethlehem, Pa., a corporation of Pennsylvania Application January 20, 1956, Serial No. 560,360

21 Claims. (Cl. 338-48) ear or a desired non-linear voltage output as a function of shaft rotation has heretofore been suggested. However, such devices have lacked the freedom from ambiguity necessary for high precision. Accordingly, a

principal object of the present invention has been the provision of a novel and improved device of this character which provides inherent accuracy and freedom from ambiguity, especially near switching points. l

Anotherobject of the invention has been the provision of ak potentiometer device of this character which requires a minimum and constant torque.

Other and further objects, features and advantages of the invention will appear more fully from the following description of one embodiment of the invention.l

In accordance with the invention, these objects are achieved by providing a controllable function potentiometer, comprising a switching mechanism having a plurality of contacts, potentiometer means having a plurality of ltaps which may be spaced in accordance with a predetermined function and each connected to a respective one of said contacts, an interpolating potentiometer having 'a iirst wiper movable relative to the winding thereof "tva'ry the interpolating potentiometer resistance between'a minimum value and a maximum value, a wiper element movable relative to said contacts and having a` pair of wiper contacts arranged to engage successively adjacent pairs of said contacts, each pair including one of the contacts of the preceding pair, said relative motion of said first wiper and said relative motion of said wiper element being interrelated so that said `interpolating potentiometer resistance varies between minimum and maximum during the interval that said `wiper element contacts engage one -pair of said contacts,

means lto produce said relative motions, and means operative upon cessation of said relative motions to cause said wiper element to assume a predetermined position relative to an adjacent pair of contacts and in which said wiper contacts are in contact with an adjacent pair of said contacts, -said predetermined position relative to said adjacent pair of contacts being independent of the particular pair of contacts forming said adjacent pair.

The invention will now be described in greater detail with reference to the appended drawings, in which:

Fig. 1 is a diagrammatic representation of `a controllable function potentiometer illustrating the principles of operation thereof;

Fig. v2 is an enlarged detail view, partly in cross-section, of a portion of Fig. l;

Fig. 3 is a front elevational view of a controllable function potentiometer constructed in accordance with the invention;

Fig. 4 is a developed Vview ofthe dial provided in the potentiometer device of Fig. 3;

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2 Fig. 5 is a side elevational View of the potentiometer device of Fig. 3;

Fig. 6 is a longitudinal sectional view taken along'the Fig. 7 is a detail view, partly in cross-section, of the main shaft and worm of Fig. 3, with the mechanism at rest;

Fig. 8 is a detail view, similar `to Fig. 7, but showing the shaft and worm during rotation with the worm engaging the worm gear;

Fig. 9 is a detail view of the face of the worm of Figs. 7 and 8;

Fig. `l0 is a side view of the cam stop of Figs. 7 and 8; and

Fig. l1 is a front view of the cam stop of Fig. l0.

As will be evident, the present invention has utility in a variety of situations where an output voltage or resistance which varies as a predetermined function of shaft rotation is desired. A particularly advantageous application ofthe invention is in a system such as is disclosed'in the copending patent application of Charles W.

'Hargens et al. entitled Data Conversion System, Serial No. 560,341, filed January 20, l1956. In this co-pending application there is described a system for converting the output impulses of a direct reading spectrograph, vfor example, one of the type described in Saunderson et al.

Patent 2,577,814, granted December ll, 1951, or in Geifner et al. Patent 2,446,874, granted August 10, 1948, into a printed analysis of the sample subjected to spectrographic analysis. Data received from a spectro'graph such as the one described in the above mentioned Saunderson et al, and Geffner et al. patents, may be in the form of electrical gates whose lengths represent concentration ratios, i.e., ratios of concentration of particular elements to a reference element in the sample. The time durations of the electrical gates are converted into specific values of some parameter, such as voltage or Vresistance in an electrical network. In general, the relationship between time duration of a spectrographic output gate'and the corresponding concentration ratio will be non-linear, i.e., will follow some empirical curve, which may be established through well known calibration techniques. Thus, for each `increment of ktime duration of an output pulse there will be a corresponding resistance or voltage value proportional to the concentration ratio.

Basically, the controllable function potentiometer with which the present invention vis concerned comprises a voltage divider with adjustable taps connected to the contact points of a switching device, a linear interpolating potentiometer connected to the wipers of the switching device, and a mechanical drive for operating these elements. In the diagrammatic representation of Fig. l, the voltage divider or main potentiometer is shown at 20, the switching device at 21, the linear interpolating potentiometer at 22, and the mechanical drive at 23.

The potentiometer 20 is provided with a number of taps suitably spaced `along its winding and designated O, l, 2, 3, 59 and 60. intermediate taps are, of course, provided but these are omitted from the drawing for clarity. The location of the taps should be selected in accordance with the predetermined function, and preferably each successive tap will correspond to an equal time increment, shaft angle or other input increment. However, theresistance values between successive taps will vary in accordance with the functional relationship, e.g., the functional relationship of concentration ratio to time. One end of the potentiometer 20 may be coupled to a regulated power supply 24 through an adjusting potentiometer 25. The power Supply 24 may be either AiC. or D.C., although it will preferably be D.C. The other end of the potentiometer 20 is coupled to ground through an adjusting potentiometer 26.

The switching device 21.shown developed in Figs. 1 and 2 may comprise two circular switch plates 27 and 28, each provided with a number of contact points 29 arranged so that the contact points on onel of the switch plates are staggered from the contact points on the other plate. The contact points on the switch plate 28 may be connected to successive even numbered taps on the potentiometer 20, while the contact points of the switch plate 27 may be connected to odd numbered taps on the potentiometer `20. Thus, the contact points 0, 2 and 60 of the switch plate 28 are shown as being connected to the taps 0, 2 and 60, respectively, of the potentiometer 20, while the contact points 1, 3 and.59 of the switch plate 27 are shown as being connected to the taps l, 3 and 59, respectively, of the potentiometer 20.'

A wiper-carrying arm 30 is mounted on a Vgear 31 which is arranged to mesh with a worm 32 carried by a shaft 33. A wiper 34 carried on the arm 30 is arranged successively to engage the Contact points on the switch plate 28 as the arm 30 is rotated. Similarly, a wiper 35 carried on the arm 30 is arranged successively to engage contact points of the plate 27. The wipers 34 and 35 are insulated from each other. As will appear most clearly from Fig. 2, during a portion of the travel of the wiper 35 past the contact connected to the tap l, the Wiper 34 will make with the contact connected to the tap 0. During a later portion of the travel of the wiper 35 past the contact connected to tap l, the wiper 34 will make with the contact connected to tap 2. The staggered relationship of the contacts on the plate 27 and the contacts on the plate 28 causes this `alternate connection to contiue as the arm 30 moves along.

lPor convenience, only a few of the taps on the potentiometer 20 have been illustrated. VEach of the taps ,should be adjustable to Iaccommodate different characteristic curves, and each tap will be set at a resistance position on the potentiometer 20 corresponding to the height of the characteristic curve, having regard to the time elapsed between the Zero position of the arm 30 and its position when one of its rwipers is connected to any particular tap of the potentiometer 20. The resistance values between adjacent taps along the potentiometer 20 will, in general, be diierent. By making these taps adjustable, any particular characteristic curve can easily be simulated. Since only a iinite number of taps is provided on the potentiometer 20, this potentiometer together with its associated interpolating potentiometer 22, will be equivalent to an approximate curve consisting of chords joining p'oints along the characteristic calibration curve. As the number of taps provided is increased, the accuracy achieved will be increased. The resistance between adjacent taps on potentiometer 20 will correspond to the difference in amplitude between adjacent taps on the calibration curve.

With any given number of taps on the potentiometer 20, accuracy can be increased by providing the interpolating potentiometer 22 with ner resolution. The interpolating potentiometer 22 may have a pair of equal resistance elements 36 and 37 arranged to be wiped by a rotating contact 38 carried on the shaft 33. Junction points 36 and 37' of the resistance elements 35 and 37 are 'connected to wipers 35 and 34, respectively. A second rotating contact 39, which is directly connected to and rotates with the contact 38, is arranged to wipe a conductive ring 40 which is connected to an output terminal 41.

The shaft 33 is arranged to be rotated by a motor 42, which is preferably of the constant speed type, such as a synchronous motor. An electric clutch 43 is provided operatively to connect the motor 42 and the shaft 33 when a signal is applied to clutch input terminals 44. A brake mechanism 45 is preferably provided positively to stop the shaft 33 when the clutch 43 is disengaged at the end of the signal applied to the terminals 44. For

this purpose, the brake -45 is provided with signal input terminals 46.

When the motor 42 and the clutch 43 are energized, the shaft 33 will be rotated in the direction illustrated, carrying with it the Worm 32. The worm 32 meshes with gear 31, in turn rotating the arm 30 from a zero position in which wipers 34 and 35 engage the contacts 0 and l, respectively, toward the right in Fig. l. In order to provide ilexibility, more contact points may be provided on the switch plates V27 and 28 than taps are provided on the potentiometer 20. in this way different potentiometers 20 may be accommodated. Similarly, not all of the taps of the potentiometer 20 need be employed if only a portion of its total resistance is required to approximate the particular functional relationship being considered. Y

As the arm 30 is driven forward by the motor 42, the interpolating potentiometer is switched sequentially from one pair of taps of the potentiometer 20 to the next, such next pair including one of the taps of the previous pair. As will be seen from the connections in Fig. l, the interpolating potentiometer 22 is connected between the Vtaps 0 and l, so that the resistance between output terminal 41 and ground :will be the resistance between tap 1 and ground plus the parallel resistance of two-branches, a rst extending between junction point 36 and wiper 3S, and a second including the resistance of potentiometer 20 between taps 0 and l and the Vresistance between junction point 37' and wiper 38. It is evident that this resistance will be somewhere between the resistance to ground ot-the tap 0 and the resistance to ground of the tap l. The exact value of this resistance between output terminal 41 and ground will be dependent upon the position of the wiper 38 along the resistor 37. Preferably, the wiper 38 of the interpolating potentiometer 22 rotates for each pair of contact points on potentiometer 20, e.g., in one complete revolution of shaft 33 the contact arm 30 will move so that wipers 34 and 35 will advance from 0 -to 1 position, respectively, to 2 tor3 position', respectively. At 180 of rotation, the wipers 34 and 35 will rest von taps 2 and l, respectively, at 360, wipers 34 and 35 will rest on 2 and 3, respectively. ln like manner, contact arm 30 will rotate so that when the wipers 34 and 35 `are positioned on 0 and l, respectively, wiper 38 will be positioned between 37 and 36. At 180, when wipers 34 and 35 are located on taps 2 and l, respectively, Wiper 38 will be positioned between 36 and 37. At 3609, when'wipers 34 and 35 are located on taps 2 and 3, respectively, wiper 38 will again be positioned between From this it is seen that when the arm 30 is located so that wipers 34 and 35 engage taps 0 and 1, respectlvely, wiper 38 can be positioned anywhere between junction points 37 and 36. At the zero position or starting position of the device being described, wiper 38 will engage junction point 37, and the resistance between outputV terminal 41 and ground will be exactly the resistance between tap 0 and ground. As shaft 33 rotates, wiper 38 will approach junction point 36. When wiper 3S exactly reaches junction point 36', wipers 34 and 35 will be positioned on taps 2 and 1, respectively. The resistance between terminal 41 and ground will have decreased so that it is now exactly the Vresistance between tap l and ground. While wipers 34 and 35 are located on taps 2 and 1, respectively, wiper 38 can be positioned, by rotation of shaft 33, anywhere between junction points 36' and 37. The ratio between worm 32 and gear 311 and the spacing between adjacent contacts 29 should be selected so'that the arm 30 advances one contact pitch while wiper 33 makes one-half a revolution.

While the interpolating potentiometer 22 is connected toV anyV one pair of taps, its output or resistance value varies linearly with time. However, upon moving to another pair of taps the rate of variation changes so as to approximate the shape of the calibration curve. The

yreason'for this changeV of rate lies in the fact that the resistance between adjacent taps of potentiometer 20 vis included in the resistance variation with time, and

the resistance between any adjacent pair of taps will, in general, be different from the resistance between any other adjacent-pair of taps. When a suicient number of taps is provided on thepotentiometer 20, the curve vwill be smooth enough to give a desired accuracy to the functional relationship. Because of the application of potential from power supply 24 to potentiometer 20 through potentiometer 25, the voltage between terminal l41 and ground will vary in the same manner as the re- -sistance between terminal 41 and ground.

The tapsettings of the potentiometer 20 may easily be shifted to accommodate changes in the calibration curve or to accommodate a totally different calibration curve. In order to prevent crowding of the taps on the potentiometer 20, which might occur, for example, when the characteristic curve is of a logarithmic character, it has been found desirable in some circumstances to break the potentiometer 20 up into sections differing in resistance` pitch. -This permits spreading the windings out and facilitates placing the taps at the desired locations without having them interfere with each other or with the winding terminals. Alternatively, the potentiometer 20 may be realized as a number of individual potentiomleters, one end of each of ywhich `is connected to the wiper of the preceding potentiometer so that each individual potentiometer will correspond to the resistance `between adjacent taps on the potentiometer 20, as illusytrated in Fig. 1.

Referring now particularly to Figs. 3, and 6, there are shown elements forming a mechanical embodiment of the controllable function potentiometer of Figs. l and 2, with the exception of the potentiometers 20, 25v and 26 and the power supply 24, the connections vof which to the mechanism illustrated in the other figures will be obvious. For convenience, the brake 45, the clutch 43 and the synchronous motor 42 are shown only in Fig. 6. The device is carried on a frame member designated vgenerally as 50 and having legs 51, 52, S3 and 54.

The frame 50 isprovided with oppositely disposed upwardly extending projections 55 and 56, each of lwhich is provided with a centrally located circular hole. A bushing 57 tits within the hole in the projection 55 and is provided with a flange 58 which locates the bushing accurately with respect to the wall of the projection 55. The other endof the bushing 57 is threaded to receive a nut 59, which, with the flange 58, holds the bushing 57 firmly in place. A threaded plug -60 lits into the outer portion of the bushing 57. The plug 60 is provided with a slot 61 adapted to receive a screw driver for adjusting the position of the plug 60 in the bushing 57. The inner end of the plug 60 is provided with an opening into which projects an end 63 of a shaft 64. The shaft 64 is journaled for rotation in a bearing 62 which is carried in the bushing 57 at a position determined by the location of the plug'60. The shaft 64 is supported in similar members provided in the projection 56'. A gear 67 (corresponding to the gear 31 of Fig. l) is mounted on the shaft 64 by means of a set screw 63 acting in a hub portion 68 of the gear 67.

A switch plate 70 (corresponding to the plate 27 of Fig. 1) is held against the projection 55 by the nut 59. A switch plate 71 (corresponding to the switch plate 2S of Fig. l) is held against the projection 56 by a similar nut. The switch'plates 70 and 71 carry contacts 72 (corresponding to the contacts 29 of Fig. l). A wiper arm 73 (corresponding to the arm 30 of Fig. l) is mounted adjacent the periphery of the gear 67 by means of a screw 74. The arm 73 carries wipers 75 and 76 (correslpending, respectively, to the wipers 35 and 34 of Fig. l),

which wipers are adapted to engage the Various contacts 72 as the gear 67 is rotated.

The gear V67 isaIranged to. `berotated by a double 6 threaded worm (corresponding to the worm 32 of Fig. 1), which may be integral with or suitably mounted on a hollow shaft 81. A bushing 82 is carried on and rotates with a main shaft 83. The bushing 82 is provided with an axially extending tongue portion 82', which is adapted to extend into a slot provided in the shaft 81 and engages the walls of the slot to cause the shaft 81 to rotate with the shaft 83. A bushing 82A is slidably mounted on a hub portion 83' of the bushing 82. One end of the bushing 82A abuts against the adjacent end of the shaft 81. The shaft 83 (which corresponds to the shaft 33 of Fig. l) is journaled for rotation in a bearing 84 provided in the frame 50. The shaft 83 extends through the bearing 84 and into a chamber 85 formed by a housing 86 aixed to the frame 50 'by screws S7. An interpolating potentiometerr'SS (corresponding to the potentiometer 22 of Fig. 1) is affixed to the housing 86 by clamps 89 and screws 89" to permit rotation of the potentiometerSS for adjustment purposes.

Within the chamber the shaft 83 is rigidly coupled to a shaft 90 by a coupling I91. The shaft 90, which extends through the interpolating potentiometer 88, r'otates wipers thereof (corresponding to the wipers 38 and 39 of Fig. l). The outer end of the shaft 90 passes through the brake 45 and in the clutch 43 Vis arranged for selective operational engagement with the shaft of the synchronous motor 42.

An outwardly extending projection 92 of the bushing 82A and an outwardly extending shoulder 93 of the bushing 82 are arranged to engage opposite ends of a coil spring 94 carried on the hub portion 83.

The shaft 83 is also journaled for rotation in a bearing 96 and a-bushing 97 carried-in a hollow bushing 98, which is aiixed to the frame 50 by screws 99. The outer end of the shaft 83 carries a hub extension 100 of a dial 101, as best shown -in Fig. S. The dial 101 is recessed so as not to interfere with the heads of the screws 99. As shown in Figs. 4 and 8, the dial 101 may be graduated to indicate the rotational position of the shaft 83 and the worm 80. The graduations are shown in two groups, each covering 180, since each rotation of the shaft 83 corresponds to travel of the wiper arm 73 past two successive pairs of contacts, e.g., contacts 0 and l and contacts l and 2 of Fig. 1. Since the position of the dial 101 is directly proportional to the interpolating potentiometer setting, this dial is very useful in Calibrating.

The inner end of the bushing 98 is formed as a laterally extending flange 102, having projecting portions 103 and 103' adapted to-contact the ends of the threads of the worm 50, as best shown in Figs. 7 and 8.

The ange 102 isprovided with peripheralslots through which, as shown in Figs. 6 and 8, passes a yoke 104', which is an integral part of arm 104. The one end of arm 104 is affixed to a bracket 105 by means of screws 105. The other end of the arm 104, which contains yoke 104', is bent so as toengage the end of a bushing 81 which is slidably mounted on shaft 83. The other end of bushing 81 engages the end of shaft 81. The arm 104 is arranged to be attracted toward the right (Fig. 6) upon energization of a solenoid 106, which is also mounted on bracket 105. Bracket 105 is affixed to legs S1 and 54 by means of screws 106. When the solenoid 106 is energized, the arm 104 pushes the bushing 81 in a direction to move shaft 81 so as to compress the spring 94 and to prevent the worm 80 from engaging the projections 103 and 103. When the solenoid 106 is denergized, the spring 94 pushes the shaft 81 and bushing 31 to the left (Fig. 6) until the worm teeth engage the cam-like projections 103 and 103 of the bushingor cam stop 9S. As will be seen in Fig. 10, which is a side View of the cam stop 98, the projections 103 and 103 are spaced apart and the worm-engaging ends thereof are shaped so as to permit flush engagement of the worm teeth with the projections. The solenoid 106 can be replaced with any suitable mechanism for axially moving the worm. A

The Worm 80 should be movable along the lateral axis of the shaft 83 by an amount equal to one thread pitch of the worm. Since each 180 of worm rotation drives the worm gear 67 and the wiper arm 73 by an amount equal to the distance between adjacent opposite switch plate contacts, the lateral movement of the worm 80 can rotate the worm gear 67 by a maximum amount equal to the distance between opposite adjacent switch plate contacts.

When the solenoid 106 is released, which will correspond in time to disengagement of the clutch 43 and application of the brake 45, the actual lateral movement imparted to the worm 80 (and hence rotary movement imparted to the worm gear 67) under action of the spring Y94- will depend upon the rotational position of the worm 80. The reason for this is that the helical worm threads, which are adapted to abut against the cam projections 103 and 103 of the cam stop 98, act in the `manner of a cylindrical cam. The worm will thus take a lateral position determined by its angular position relative to the stationary stop 98. Twice during each revolution of the drive shaft 83, the ends of the worm threads coincide with the points of the projections 103 and 103 of the cam stop 98. These times represent the maximum possible lateral travel of the worm and correspond to situations in which one of the wipers 34 and 35 (Fig. 2) engages one of the switch plate contacts, while the other is just about to come into engagement with the next switch plate contact. For example, in Fig. 2, considering wiper movement from left to right, maximum Worm lateral travel could occur at a position just in advance of either the solid or dotted line wiper positions shown.

The mechanics by which the worm gear'67 is rotated so as to move the wiper arm 73 past the contacts will now be explained. Let it be assumed that the worm is lin the position shown in Fig. 6,'with its teeth engaging the cam stops 103 and 103 and at their extreme leftward position. (In other words, in a position in which the teeth have just passed by the crest of the cam stops and have been pushed axially to the left by spring 94.) In this position, the wiper contacts 75 and 76 `on arm 73 will each be in engagement with one of the contacts 72 on the plates 70 and 71. For the sake of simplicity, it will be assumed that the solenoid is not energized.

If, under these conditions, shaft 81 is rotated counterclockwise (when viewed from the right-hand end as shown in Fig. 6), the worm 80 will tend to rotate worm gear 67 in a clockwise direction as viewed in said Fig. 6. However, as Vworm 80 rotates on its axis it is simultaneously moved to the right by reason of its teeth riding up on the cam stops 103 and 103. This movement of worm 80 axially to the right tends to rotate the worm gear counterclockwise as shown in Fig. 6. Since the pitch of cam stops 103 and 103 is the same as that of worm 80, the net result'is that the two forces tending to rotate worm gear 67 cancel each other out and no rotation of worm gear 67 results until the teeth of worm S0 pass beyond the crests of stops 103 and 103', at which point the worm 80 jumps to the left by reason of spring 94 and simultaneously the worm gear 67 jumps in a clockwise direction.

As shaft 81 continues to turn, the operation above described is repeated, with the result that the worm gear 67 is given an intermittent rotation in a clockwise direction.

The pitch of worm 80 is so selected that each inter- -mittent movement of gear 67 moves the arm 73 and the wipers carried thereby from one pair of contacts 29 to the next succeeding pair (as from the position shown in full lines in Fig. 2 to the position shown in broken llines in Fig. 2).

The result is that no matter at what position the shaft 81 stops rotating the contacts 34-and 35 on the wiper arm will always be in engagement with a pair of contacts 2,9. The wiper arm cannot stop in a position in which one of the wiper contacts 34 or 3 5 is between a pair of the contacts 29. i

lt will be understood that while the shaft 81 is turning through 180 as abovev described, with the worm gear remaining stationary, the wiper arm of the interpolating potentiometer 88 (which arm is mounted on shaft 81) is swept through 180, and as the interpolating potentiometer is connected to contacts 34 and 35 the resistance through the circuit is increased linearly. As soon -as the shaft 81 has revolved through 180 the teeth of worm pass beyond the crests of stops 103-103, the worm jumps to the left, the worm gear 67 jumps clockwise, the contacts on wiper 73 jump to the next pair of contacts 29, and the wiper arm of the interpolating potentiometer 88 moves past one of the taps 36 and 37 and the reactance imposed by the interpolating potentiometer falls to zero. The net result of this arrangement of main potentiometer and interpolating potentiometer is that its output is in the form of a plurality of chords connecting the points of the curve for which the main potentiometer is calibrated.

In the above description it has been assumed thatthe solenoid 106 has not energized, with the result that worm 80 is at all times urged into contact with stops 103 and 103' by spring 94. This is aperfectly feasible method of operation but results in an uneven torque against motor 42, and unnecessary wear on the cam and the face of the worm, such wear in time leading to inaccurate discrimination of the switching point. Therefore, I nd it very desirable to provide solenoid 106 and arm 104, and toV energize the solenoid at all times while the motor 42 is operating. Energizing the solenoid 106 moves arm 104 to the right, as shown in Fig. 8, which pushes sleeve 81 and worm 80 to the right, out of contact with stops 103 and 103. Motor 42 turns shaft 81, and as the worm 80 and stops 103 and 103 are out of contact with each other, worm gear 67 rotates continuously in a clockwise direction, instead of intermittently as above described. When the shaft 31 stops turning, means are provided to automaticallyl de-energize the solenoid 106, at which time spring 94 pushes worm 80 to the left and into contact with stops 103 and 103. The amount of movement of worm 80 to the left will depend on its angular position.V However, it will be understood from the previous description that because of the conguration of the worm 80 and stops 103 and 103 the worm 80, and hence the gear 67, will automatically assume a position in which both of the contacts on arm 73 are in engagement with a pair of contacts 29. By using solenoid 106 and arm 104 to move the worm S0 and stops 10S-103' out of engagement while shaft 81 is rotating the torque imposed on motor 42 is kept uniform, thus contributing to the accuracy of the apparatus.

It is important to observe that although the worm and the worm gear both move under the action of spring 94, the interpolating potentiometer 88 (22 in Fig. l) is not moved by this action, so that its setting remains unchanged. Because of this, the resistance between terminal 41 and ground will be established accurately no matter what the rotational position of the worm when it is stopped.

Establishment of the proper resistance value without ambiguity by lateral motion of the worm limited by the cam stop, as described, permits operation with a minimum and constant torque. Ambiguity elimination, as by means of a Geneva movement, would cause peaks of torque requirement.

While the cam stop is shown in Figs. l0 and ll a's being provided with two worm-engaging or cam-follower faces 103 and 103 in order to accommodate a double threaded worm, only one Worm-'engaging surface would be required for a single threaded cam. With a single VAtion ranges.

threadedk cam, however, it would vbe desirable to have y360" of worm rotation (rather than 180) drive the worm Agear andthe wipers one contactpitch. Multiple threaded Worms can be used, if desired, with appropriate modifications.

Should the signal input to terminals 44 exceed the calibration range of the controllable function potentiometer', it is desirable that the clutch 43 be disengaged and the brake 45 operated. For this purpose, there is provided a maximum travel limit switch 110 (Fig. 5) which,

when operated, may disengage the clutch 43 and operate the brake 45, as explained in the above mentioned copending application of C. W. Hargens et al. Switch 110. is arranged tobe operated by a pivoted arm 111, an end 112 of which is located in the path of a pin 113 mounted on an arm 113 carriedon the shaft 64. The location of the arm 113 relative to `the wiper-carrying arm 73 may be adjusted by loosening set screws 114 by vwhichthe arm 113 is axed tothe shaft 64. Such ad` justment is ,desirable to accommodate different calibraf desired, the switching mechanism may be arranged for continuous rotation. In such case, the maximum travel limit switch and associated apparatus would not be'provided, or Ameans would be provided to render them inoperative. Also, the wiper-carrying arm would have to be arranged so as not to strike the worm. In order to assure accuracy each time the controllable function potentiometer has lbeen used, it should be returned precisely to its zero setting. ln order to return to zero, the solenoid 106 should be energized, the clutch 43 should be engaged, and the brake 45 should be released. The motor 42 should be energized in the reverse direction, ora separate return travel motor should be provided. During the point in the return travel at which the wipers 75 and 76 are a predetermined number of contact segments` away from their zero position (pref- Verably one contact segment), a stud 115, which is mountedV near the Yperiphery of the worm gear 67, engages an end 116Y of afpivoted arm 117 (Fig. 5) and causes Ithe latter to operate -a limit switch 118. The switch 118 may be used to set up a stopping circuit and, ifdesired, to slow the return travel motor so that precise stopping at the zero position may be achieved. Operation of theswitch 1118 is an indication that the zero position is within one complete worm rotation. An indication of theexact zero position is provided by operation of a switchz119 (Fig. 3) by a'spring arm 120 which carries a follower 121 in engagement with a cam 122. As bestshown in Fig. 8, the shape of the cam 122 is such that the follower 121 and the arm 120 will operate the switch 119once during each revolution of the worm. For this purpose, the cam-122 is aixed to the dial 101 by means of screws 123 acting in elongated slots provided in the cam 122. Provision of the elongated slots permits shifting of Ithe cam 122 relative to the dial 101 so that the zero position may be adjusted accurately.

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur .to those skilled in the art without departing from the spirit and scope of the invention, as set forth in the appended claims.

What is claimed is:

1. A controllable function potentiometer, comprising a switching mechanism having a plurality of contacts, potentiometer means having a plurality of taps which may be'spaced in accordance with a predetermined function and each connected to a respective one of said contacts, an interpolating potentiometer having a rst wiper movable relative to the Winding thereof to Vary the interpolating potentiometer resistance between a minimum value and a maximum Value, `a Wiper element movable relative to said contacts and having a pair of wiper contacts arranged to engage successively adjacent pairs of said contacts, each pair including one of `the contacts of the preceding pair, said relative motion of said'rst vwiper and said relative motion of said wiper 'element pair.

2. A controllable function potentiometer, comprising a switching mechanism having a plurality of contacts,

`potentiometer means having a plurality of taps which may be spaced in accordance with a predetermined function and each connected to a respective one of said contacts, an interpolating potentiometer havingra first wiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between a minimum value and a maximum value, a wiper element movable relative to said contacts and having a pair of wiper contacts arranged to engagesuccessively adjacent pairs of said contacts, each pair including` one of the contacts Vof the preceding pair, a worm gear arranged to move said wiper element, a worm in operative engagement with said worm gear, a driving mechanismfor synchronously rotating said worm gear and said rst wiper whereby Ysaid relative motion of said iirst wiper and said relative motion of said wiper element are interrelated so that said interpolating potentiometer resistance varies between minimum and maximum during the interval that said Wiper element contacts engage one .pair of said contacts, means to operate saidV driving mechanism, and means Voperative upon stopping of said driving mechanism to cause axial movement of said worm by an amount dependent on the rotational position of said worm thereby to cause said wiper element to assume a predetermined position relative to an adjacent pair of said contacts .and in which said wiper contacts are in contact with said ad- .jacent pair, said predetermined position relative tosaid adjacent pair of contacts being independent of the particular Apair of contacts forming said adjacent pair.

3. A controllable function potentiometer, comprising a switching mechanism including a first group of spaced contacts, a second group of spaced contacts and a movable wiper element having a pairof wiper contacts each arranged successively to engage the contacts of a respective one ofsaid groups, the contacts of one group being staggered relative to the contacts of the other group-so thatsaid wiper contacts engage successively adjacent pairs of said contacts, each pair including one of the contacts of the preceding pair; potentiometer means having aplurality of taps which may be spaced in accordance with a ytive engagement with said worm gear; an interpolating potentiometer having a rst wiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between a minimum value and a maximum value; a driving mechanism for synchronously rotating said worm and said lirst wiper thereby to interrelate the motions of said first Wiper and said wiper element whereby said interpolating potentiometer resistance varies between minimum and maximum during the interval that said wiper element contacts engage one pair of said contacts; means selectively to operate said driving mechanism; and means operative upon stopping of said drivingmechanism to cause axial movement of said worm by an amount dependent on the rotational position of said worm thereby to cause said wiper element to assume a'predetermined position relative to an adjacent pair-of said contacts and in which said wiper contacts are in contact with said adjacent pair, said predetermined position relative to said adjacent pair of being independent of the particular pair of contacts forming said adjacent pair.

4. A controllable function potentiometer, comprising a switching mechanism including a irst group of spaced contacts, a second group of spaced contacts and a movable wiper element having a pair of wiper contacts each arranged successively to engage the contacts of a respective one of said groups, the contacts of one group being staggered relative to the contacts of the other group so that said wiper contacts engage successively adjacent pairs of said contacts, each pair including one of the contacts of the preceding pair; potentiometer means having a plurality of taps which may be spaced in accordance with a predetermined function and each connected to a respective one of said contacts, successive taps being connected to contacts of a different group; a worm gear arranged to move said wiper element; a worm in operative engagement with said worm gear; an interpolating potentiometer having a first Wiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between a minimum value and a maximum value; a driving mechanism for synchronously rotating said Worm and said first wiper thereby to interrelate the motions of said first wiper and said wiper element whereby said interpolating potentiometer resistance varies between minimum and maximum during the interval that said wiper element contacts engage one pair of said contacts; means selectively to operate said driving mechanism; means operative upon stopping of said driving mechanism to cause axial movement of said worm Vby an amount dependent on the rotational position of said worm thereby to cause said wiper element to assume a predetermined position relative to an adjacent pair of said contacts and in which said Wiper contacts are in contact with said adjacent pair, said predetermined position relative to said adjacent pair being independent of the particular pair of contacts forming said adjacent pair; and circuit means intercoupling said interpolating potentiometer and said wiper contacts to provide an output resistance which varies in accordance with said function.

5. A controllable function potentiometer, comprising la switching mechanism including a first group of spaced contacts, a second group of spaced contacts and a movable wiper element having a pair of wiper contacts each arranged successively to engage the contacts of a respective one of said groups, the contacts of one group being staggered relative to the contacts of the other group so that said wiper contacts engage successively adjacent pairs of said contacts, each pair including one of the contacts of the preceding pair; potentiometer means having a plurality of taps which may be spaced in accordance with a predetermined function and each connected to a respective one of said contacts, successive taps being connected to contacts of a diiferent group; a worm gear arranged to move said wiper element; a Worm in operative engagement with said worm gear; an interpolating potentiometer having a first wiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between la minimum value and a maximum value; a driving mechanism for synchronously rotating said worm and said rst wiper thereby to interrelate the motions of said first Wiper and said wiper element whereby said interpolating potentiometer Vresistance varies between minimum and maximum during the interval that said wiper element contacts engage one pair of said contacts; means selectively to operate said driving mechanism; spring means arranged to urge said worm to move in an axial direction; means to prevent axial movement of said worm While said driving mechanism is operating; a stop interposed in the path of axial movement of said worm to limit said axial motion thereof to an amount dependent on the rotational position of said worm thereby to cause said wiper element to assume a predetermined position relative to an adjacent pair of said contacts and in which said wiper contacts are in contact with said adjacent pair, said predetermined position relative to said'adjacent pair being independent of the particular pair of contacts forming said adjacent pair; and circuit means 'intercoupling said interpolating potentiometer and said wiper contacts to provide an output resistance which varies in accordance with said function.

6. A controllable function potentiometer, comprising a switching mechanism including a first group of spaced contacts, a second group of spaced contacts and a movable wiper element having a pair of wiper contacts each arranged successively to engage the contacts of a respective one of said groups, the contacts of one group being staggered relative to the contacts of the other group so that said wiper contacts engage successively adjacent pairs of said contacts, each pair including one of the contacts ofthe preceding pair; potentiometer means having a plurality of taps which may be spaced in accordance with a predetermined function and each connected to a respective one of said contacts, successive taps being connected to contacts of a different group; a worm gear arranged to move said wiper element; a double-threaded worm in operative engagement with said worm gear; an interpolating potentiometer having a rst wiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between a minimum value and a maximum value; a driving mechanism for synchronously rotating said worm and said first wiper thereby to interrelate the motions of said first wiper and said wiper element whereby said interpolating potentiometer resistance varies between minimum and maximum during the interval that said wiper element contacts engage one pair of said contacts, said driving mechanism including a shaft arranged to rotate said worm but permitting axial movement of said worm; means selectively to operate said driving mechanism; a spring arranged to engage said Worm and to urge the latter to move in an axial direction; means to prevent axial movement of said worm while said shaft is rotating; a cam stop having a pair of projections interposed in the path of axial movement of said worm and each arranged to engage a respective thread of said worm to limit axial movement thereof to an amount dependent on the rotational position of `said worm thereby to cause said wiper element-to assume a predetermined position relative to an adjacent pair of said contacts and in which said wiper contacts are in contact with said adjacent pair, said predetermined position relative to said adjacent pair being independent of the particular pair of contacts forming said adjacent pair; and circuit means intercoupling said interpolating potentiometer and said wiper contacts to provide an output resistance which varies in accordance with said function.

7.V ln a controllable function potentiometer device, the combination comprising a switching mechanism, a worm gear arranged to operate said switching mechanism, a worm in operative engagement with said worm gear, means selectively to rotate said Worm thereby to rotate said worm gear and operate said switching mechanism, and means operative upon cessation of rotation of said Worm to move the latter in an axial direction by an amount dependent on the rotational position of said-worm when said rotation thereof ceases,

S. in a controllable function potentiometer device, the combination comprising a switching mechanism, a worm gear arranged to operate said switching mechanism, a worm having helical threads in operative engagement with said Worm gear, means selectivelyV torotate said worm thereby to rotate said worm gear and operate said switching mechanism, a spring arranged to move said worm in an axial direction, a cam stop disposed in the path of axial movement of said helical threads to limit axial movement of said worm to an amount dependent on the rotational position of said worm, and means operative during rotation of Ysaid wormto prevent said axial movement thereof.

9.Y In a controllable function potentiometer device, the combination comprising a switching mechanism, a worm gear arranged to operate said's'witching mechanism, a worm having double helical threads-in operative engagement with said kworm gear, means selectively to rotate said worm thereby to rotate said worm gear and operate said switching mechanism, means arranged to urge said worm to move in an axial direction, means operative while said worm is rotating to prevent said axial movement of said worm, and a cam stop having two projections disposedin the paths of axial movement of said threads to limit said` axial movement to an amount dependent on the rotationalposition of said' Worin when the latter stops rotating.

l; A controllable function potentiometer, comprising a switching mechanism including a iirst group of spaced contacts, a second 'group of spaced contacts and a movable' wiper element having a ypair ofI Wiper contacts each arranged successively to engage the contacts of a respective one of said groups, the contacts -of one group being staggered relative to the contacts of the other group so that said wiper contacts engage successively adjacent pairs of said contacts, each pair including one of the contacts of the' preceding pair; potentiometer means having a plurality of taps which may be spaced in accordance with a predetermined function and'each connected to a respective one of said contacts, successive taps y being connected to Vcontacts of a different group; a worm gear connected to said wiper element for moving the latter from a predetermined starting position; a worm in operative engagement with saidrworm gear; an interpolating potentiometer having a iirstwiper movable relative to the winding thereof to vary the interpolating potentiometer resistance between a yminimumvalue anda maximum value; a driving mechanism for synchronously rotating said worm and said first wiper thereby to interrelate the motions of said iirst wiper and lsaidwiper element whereby said interpolating potentiometer resistance varies between and-maximum during vthe interval that said wiper element contacts lengage one pair of said contacts; means selectively to operate said driving mechanism; means operative upon stopping of said driving mechanism toy cause axial movement of- -said worm by an amount dependent on the rotational yposition of said worm thereby to cause said Wiper element to assume a predetermined position relative to an adjacent'pair of lsaid contacts and in which said wiper contacts are 'in contact with said adjacent pair, said predetermined position relative to said adjacent pair being independent of the particular pair of contacts vforming said adjacent pair; means for rotating said worm in a sense to return said wiper element to said predetermined starting position thereof; first rswitch means arranged to be operated each time said worm is in a rotational position corresponding to its rotational lposition when `said wiper Ielement is in ysaid predetermined starting position thereof; second switch means arranged to be operated when said wiper element reaches apredetermined'position corresponding'to less than one worm rrotation from said Ypredetermined starting position; and means operativ'e upon operation of both said switchmean's for stopping said latter rotation of said worm.

l'l. A controllable function potentiometer, comprising a switching mechanism including a first group of spaced contacts, a second group `of spaced contacts and a movable wiper element having a pair of wipercontacts each arranged successively to engage thecontacts of ya `respective one of said groups, the contacts of one group. being staggered relative to the contacts of the other group so that said wiper contacts engage `successively adjacent pairs of said contacts, each pair including one of the contacts ofthe preceding pair; potentiometer,meanslhaving a plurality of taps which-may vbe spaced in accordance with a predetermined function vand each connected to a respective one. of said contacts, successive taps being connected to contacts -of a different group; aworm gear connected to said wiper element for moving the latter from a predetermined starting position; a Worm having double helical threads in operative engagement with said worm gear and arranged to rotate the latter so that said wiper contacts complete engagement with two of said pairs for each rotation of said worm; an interpolating potentiometer having a first wiper movable relative to the winding thereof to vary the interpolating vpotentiometer resistance between a minimum value and a maximum value; a driving mechanism for synchronously rotating said worm and said first wiper thereby to interrelate the motions of said first wiper and said wiper element whereby said interpolating potentiometer resistance varies between minimum and maximum during the interval that said wiper :element contacts engage one pair of said contacts; means-selectively to operate said driving mechanism, means operative upon stopping of said driving mechanism to cause axial movement of said-worm; a cam stop interposed in the path of axial movement of said threads to limit said axial movement to an amount dependent on the rotational position of said worm when stopped thereby to cause said wiper element to assume a predetermined position relative to an adjacent pair of said contacts 'and in which said wiper contacts are in contact with said adjacent pair, said predetermined position relative to said adjacent pair being independent of the particular pair of contacts forming said adjacent pair; means for rotating said worm in thefopposite direction to return said wiper element to said predetermined starting position thereof; first switch means; a cam surface arranged to rotate with said worm and being shaped and located to operate said rst switch means each time said worm is in a rotational position corresponding to its rotational position when said wiper element is in said predetermined starting position thereof; second switch means; an arm arranged to rotate with said worm gear and to operate said second switch means at a predetermined position corresponding to less than one worm rotation from said predetermined starting position; and means operative upon operation of both said switch means for stopping said opposite direction rotation of said worm.

l2. A potentiometer device having a frame, comprising a regulated source of electrical power, a constant speed motor, a source of electrical current constituting a signal, a shaft rotatably mounted on the frame, a double-threaded worm splined to said shaft, a worm wheel meshing with said worm, an electrically operated clutch intermediate the motor and the shaft, any electrically operated brake mounted on the shaft, means operatively connecting said signal current source to said clutch and said brake, a primary potentiometer system having -a winding electrically associated with said power, a secondary potentiometer system having a winding arranged about the shaft, a pair of plates mounted in opposite relation at each side of the worm wheel, respectively, and coaxial therewith, a series of contactors mounted on one plate, a series of contactors mounted on the other plate in staggered relation to the first named series, a main Contact wiper mounted on said Worm wheel for engagement with said contactors successively and having a zero position with respect to a pair of oppositely disposed contactors, a pair of diametrically positioned taps in the secondary potentiometer system having a winding adapted to be incorporated in the primary system through the medium of the main contact wiper, a secondary contactor wiper electrically associated with the main contact wiper mounted to rotate with said shaft adapted 'to engage the winding of the secondary potentiometer, a series of'taps on the winding of the primary potentiometer system in operative arrangement with the contactors on one of the Vside plates, a series of taps on-the winding of the primary 15 potentiometer system arranged in operative arrangement with the contactors on the other plate, a cam surface on the worm coincident with the thread thereof, a stop surface on the frame having a surface complementary to said cam surface, a resilient power means urging said surfaces into engagement with each other, means to reverse said motor to return said main contact wiper towards the zero position, a rst limit switch to slow the return of the main contact wiper adjacent the pair of contactors next preceding the pair of contacts adjacent the zero position, a cam element mounted coaxially on the end of said shaft and angularly adjustable about the axis thereof, a follower element engaging said cam element, and a second limit switch operated by said follower element to stopv the movement of the main contact wiper at about its exact zero position, a device associated with the operation of the motor to render said resilient power means ineffective, and output terminals electrically connected to said power through the medium of said windings.

13. A potentiometer device having a frame, comprising a regulated source of electrical power, a constant speed motor, a source of electrical current constituting a signal, a shaft rotatably mounted on the frame, a worm splined to said shaft, a worm wheel meshing with said worm, an electrically operated clutch intermediate the motor and the shaft, means operatively connecting said signal current source to said clutch, a primary potentiometer system having a winding electrically associated with said power, a secondary potentiometer system having a winding arranged about the shaft, a pair of plates mounted in opposite relation at each side of the worm wheel, respectively, and coaxial therewith, a series of contactors mounted on one plate, a series of contactors mounted on the other plate, a main contact wiper mountedon said worm wheel for engagement, with said contactors successively and having aV zero position with respect to a pair of oppositely disposed contactors, a tap in the secondary potentiometer system having a winding adapted to be incorporated in the primary system through the medium of themain contact wiper, a secondary contactor wiper electrically associated with the main contact wiper mounted =to rotate with said shaft adapted to engage the winding of the secondary potentiometer, a series of taps on the Winding of the primary potentiometer system in operative arrangement with the contactors on one of the side plates, a series of taps on the winding of the primary potentiometer system arranged in operative arrangement with the contactors on the other plate, a surface on the worm coincident with the thread thereof, a stop surface on the frame, a resilient power means urging said surfaces into engagement with each other, means to reverse said motor to return said main contact wiper towards the zero position, a iirst limit switch to slow the return of the main contact wiper adjacent the pair of contactors next preceding the pair of contactors adjacent the zero position, a cam element mounted coaxially on the end of said shaftand angularly adjustable about the axis thereof, a follower element engaging said cam element, and a second limit switch operated by said follower element to stop the movement of the main contact wiper at about its exact zero position, a device associated with the operation of the motor to render said resilient power means ineffective, and output terminals electrically connected to said power through the medium of said windings.

' 14. A potentiometer device having a frame, comprising a regulated source of electrical power, a constant speed motor, a source of electrical current constituting a signal, a shaft rotatably mounted on the frame, a worm splined to said shaft, a worm wheel meshing with said worm, an electrically operated clutch intermediate the motor and the shaft, means operatively connecting said signal current source to said clutch, a primary potentiometer system having a winding electrically associated with said power, a secondary potentiometer system having a winding arranged about the shaft, a pairl of plates mounted in opposite relation at each side of the worm wheel, respectively, and coaxial therewith,I a series of contactors mounted on one plate, a series of contactors mounted on the other plate, a main contact wiper mounted on said worm wheel for engagement with said contactors successively, a tap in the secondary potentiometer system having a winding adapted to be incorporated in the primary system through the medium of the main contact wiper, a secondary contactor wiper electrically associated with the main contact wiper mounted to rotate with said shaft adapted to engage the winding of the secondary potentiometer, a series of taps on the winding of the primary potentiometer system in operative arrangement with the contactors on one of the side plates, a series of taps on the winding of the primary potentiometer system arranged in operative arrangement with the contactors on the other plate, a surface on the worm coincident with the thread thereof, a stop surface on the frame, a resilient power means urging said surfaces into engagement with each other, a device associated with the operation of the motor to renderV said resilient power means ineffective, and output terminals electrically connected to said power through the medium of said windings.

l5 .A potentiometer device having a frame, comprising a regulated source of electrical power, a constant speed motor, a source of electrical current constituting a signal,

. a `shaft rotatably mounted on the frame, a worm splined to said shaft, a worm wheel meshing with said worm, an electrically operated lclutch intermediate the motor and the shaft, means operatively connecting said signal current source to said clutch, a primary potentiometer system having a winding electrically associated with said power, a secondary potentiometer system having a winding arranged about the shaft, a pair of plates mounted in opposite relation at each side of the worm wheel, respectively, and coaxial therewith, a series of contactors mounted on one plate, a series of contactors mounted on the other plate, a main contact wiper mounted on said worm `wheel for engagement with said contactors successively, a tap in -the secondary potentiometer system having a Winding adapted to be incorporated in the primary system through the medium of the main contact wiper, a secondary contactor wiper electrically associated with the main contact wiper mounted to rotate with said shaft adapted to engage the winding of the secondary potentiometer,. a series of taps on the winding of the i primary potentiometer system in operative arrangement with the contactors on one of the side plates, a series of taps on the winding of the primary potentiometer system arranged in operative arrangement with the contactors on the otherplate, a surface on the worm coincident with the thread thereof, a stop surface on the frame for engagement with the latter surface and output terminals electrically connected to said power through the medium of said windings.

16. A potentiometer device having a frame, comprising a regulated source of electrical power, a constant speed motor, a source of electrical current constituting a signal, a rotatable element mounted on the frame, an electrically operated clutch intermediate the motor and the rotatable element, a member driven by the rotatable element, means operatively connecting said signal current source to said clutch, a primary potentiometer having a winding system, a secondary potentiometer having a winding system, a first series of contactors mounted on said frame, a second series of contactors mounted on said frame in staggered relationto the first named series, a main contact-wiper mounted on said driven member for engagement with said contactors successively, a secondary Wiper mounted on said rotatable element to engage the winding of the secondary potentiometer, a series of taps on the winding of the primary potentiometer system in p operative arrangement with the rst series of contactors,

a sen'es of taps on the winding of the primary potentiometer system arranged in operative arrangement with the second series of contactors, means to electrically associate the wipers to interpolate the resistance of the potentiometer systems in accordance with the angular position of the rotatable member, and output terminals electrically connected to said power through the medium of said windings.

17. A potentiometer including a resistance element having taps connected to a series of spaced contacts, a pair of wipers arranged to engage an adjacent pair of said spaced contacts, means for moving said wipers successively past said contacts and for positioning said wipers with respect to said contacts, said means comprising a drive shaft, means for rotating said drive shaft, a helically threaded element mounted on said drive shaft, a follower in engagement with said helically 'threaded element and carrying said pair of wipers, a stepped cam on the end of said drive shaft, a cam follower adapted to contact said cam and having a pitch equal to the pitch of said helically threaded element and a height proportionate to the distance between an adjacent pair of said spaced contacts, said drive shaft being longitudinally movable whereby contact between said cam follower and said cam effects longitudinal positioning of said drive shaft.

18. An electrical switching device including a resistance element having taps connected to a series of spaced contacts, a pair of wipers arranged to engage an adjacent pair of said `spaced contacts, means for moving said wipers successively past said contacts and for positioning said wipers with respect to said contacts, said means comprising a drive shaft, means for rotating said drive shaft, a worm mounted on said drive shaft, a worm gear in engagement with said worm and carrying said pair of wipers, a stepped cam on the end of said drive shaft, a cam follower adapted to contact said cam and having a pitch equal to the pitch of the threads of said worm and a height proportionate to the distance between an adjacent pair of said spaced contacts, said drive shaft being longitudinally movable whereby contact between said cam follower and said cam effects longitudinal positioning of said drive shaft.

19. The apparatus of claim 17, further including means effective during rotation of said shaft for urging said cam and cam follower out of engagement, and means effective on cessation of rotation of said shaft for urging said cam and cam follower into engagement.

20. The apparatus of claim 17, further including an interpolating potentiometer having a resistance element and a wiper arm, and means actuated by said shaft for moving said wiper arm over the length of said resistance element while the shaft rotates through an angular displacement equal to the angular displacement of one cam step around said shaft.

2l. An electrical switching device including a resistance element, a pair of mounting plates having opposed surfaces, a series of spaced contacts mounted on the opposed surfaces of each of said plates, the contacts on one of said plates being in staggered relationship with respect to the contacts on the other of said plates, taps connecting said resistance element to said contacts, successive taps being connected alternately and successively to contacts on opposite plates, a pair of wipers arranged to engage a single pair of contacts connected to successive taps, means for moving said pair of wipers successively past said contacts and for positioning said wipers with respect to said contacts, said means cornprising a drive shaft, means for rotating said drive shaft, a helically threaded element mounted on said drive shaft, a follower in engagement with said helically threaded element and carrying said pair of wipers, a stepped cam on the end of said drive shaft, a cam follower adapted to contact said cam and having a pitch equal to the pitch of said helically threaded element and a height proportionate to the distance between an adjacent pair of said spaced contacts, said drive shaft being longitudinally movable whereby contact between said cam follower and said cam effects longitudinal positioning of said drive shaft.

References Cited in the file of this patent UNITED STATES PATENTS 1,234,771 Kiewicz July 31, 1917 1,414,709 Reisbach May 2, 1922 2,675,451 Tinus Apr. 13, 1954 2,681,967 Harrison June 22, 1954 2,788,534 Liska Apr. 16, 1957 

